Long Term Efficiency and Stability of MX Precipitation Strengthening of High Chromium Steel

Abstract

Applicability of high Cr steels to a main structural material in fast breeder reactors (FBR) has been explored to enhance the economical competitiveness of the FBR power plants. V and Nb are believed to improve the high temperature strength of high Cr steels by precipitating as carbides and/or nitrides, namely MX fine particles, although the long-term effectiveness and stability of such a dispersion strengthening mechanism has not fully been understood yet. A series of trial melts controlling V and Nb contents are produced and aging tests are conducted to investigate the long-term stability of the MX strengthening mechanism. VX dispersion strengthening in high Cr steels bearing with V has been found to be stable even after aging for 12000 h at 600°C, which is equivalent to the expected FBR operation condition, 700000 h at 550°C, because the aspect ratio and chemical compositions of VX particles do not change much with aging. MX strengthening with Nb, and with both Nb and V, on the other hand, seems to be unstable, because the number density of MX particles decreases with aging. Z-phase is found to be stable and fine as other MX particles, suggesting that it may contribute to the dispersion strengthening as well as MX in the FBR conditions.